The present invention relates to a plastic molding system for obtaining a resin molded article having high size precision and used in, e.g., an optical system, in accordance with injection molding, a plastic injection mold for injection-molding a thermoplastic resin by using a hot runner, and a plastic molded article; and a plastic molding method for forming a boss to project from a surface of the main body of a molded article so that the molded article is connected to another article, a mold for the same, and a molded article of the same.
A conventional plastic molding system uses an injection molding apparatus and accompanying facilities. In an injection mold used in a molding system of this type, the gate of its hot runner has a section as shown in FIGS. 66 and 68.
Referring to FIG. 66, in a hot runner 40, a gate 39 is provided to be directly continuous to a cavity 38 that forms the thick-walled portion of a molded article 38A (see FIG. 67).
Referring to FIG. 68, in a hot runner 40, a gate 39 is contiguous to a cavity 38, which corresponds to the thick-walled portion of a molded article 38A (see FIG. 68), through a land portion 100 provided as a dummy.
When a plastic molded article is to be connected to another article, a boss is integrally formed to project from mainly the lower side surface, which is one surface of the molded article main body, and the connecting hole portion of the mating article is fitted on this boss, or the mating article is integrally assembled with the boss of the molded article main body through engagement of a mounting screw.
Often, a boss 150 having a shape as shown in FIGS. 70 and 71 is integrally formed on one surface of the main body of a flat molded article 38A, and the mating article is integrally assembled with this molded article 38A such that the connecting hole portion of the mating article is in slidable contact with an outer circumferential surface 200 of the boss 150. When the boss 150 is to be fitted in the connecting hole portion of the mating article in this manner, in order to maintain the gap with respect to the mating article, an annular base portion 202 is sometimes formed on the base of the boss 150, as shown in FIGS. 72 and 73. In this case, the diameter of the boss 150 is increased as compared to the wall thickness of the molded article main body. Since the base portion 202 is formed, it is known that during the cooling process the temperature drop in the boss 150 and base portion 202 is delayed as compared to the temperature drop in the molded article main body.
For this reason, when the boss 150 has a large diameter, a recessed hole 208 is formed in the central portion of the boss 150, as shown in FIGS. 74 and 75, in order to quicken the temperature drop. Especially, when the connecting portion of the mating article is to be mounted on the boss through engagement of a mounting screw, the recessed hole 208 is utilized as the screw-locking lower hole.
However, the conventional plastic molding system described above has drawbacks as follows.
(1) Usually, the temperature of factory water is about 25.degree. C. and varies by an annual average of about 3.degree. C. The temperature of the factory water at branching outlets also varies. The water temperature varies daily or hourly depending on, e.g., the number of injection molding apparatuses to which the factory water is branched or the number of machines that are simultaneously operated.
When the factory water is used as the cooling water for cooling the mold to a predetermined temperature, as the difference between the pressure of water supplied to the mold and the pressure of water returning from the mold is as small as about 1 kg/cm.sup.2 to 2 kg/cm.sup.2, the flow rate of the coolant flowing through the mold is greatly decreased by the pressure loss caused by the line resistance of the coolant and the branching line resistance in the mold. As a result, the cooling efficiency of the mold is degraded.
In order to control the temperature of the coolant in the mold, thereby stabilizing the temperature change in the mold repeated during the continuous molding processes to a predetermined pattern, a temperature controller is sometimes interposed between the mold and the cooling water supply source (central chiller). However, since the factory water usually has a temperature of 25.degree..+-.3.degree. C., as described above, the temperature of the factory water as the coolant can only be stabilized at 30.degree. C. or more by a temperature controller having only a pressure adjusting function. In addition, since the capacity of the conventional temperature controller is 60 to 80 (l/min.) at maximum discharge, when the pressure loss in the line of the coolant is 3 to 4 kg/cm.sup.2 or more, a desired flow rate cannot be actually maintained.
From these circumstances, conventionally, the quantity of heat of the resin supplied into the mold is empirically set from a cycle required for transmitting the heat by the coolant communicating through the branching lines, such that a change in mold temperature is converged to a predetermined pattern during the continuous molding processes.
More specifically, countermeasures have been taken so that the heat balance of the mold in the respective cycles, ranging from the molding temperature to the removing temperature in continuous molding, is set at equilibrium. According to this conventional system, however, the heat exchange efficiency in the cooling process is very low. Thus, the molding cycle is prolonged, and the productivity is undesirably suppressed.
(2) In the conventional molding system, data concerning an article under molding are not acquired. Therefore, injection molding is performed in advance under several kinds of molding conditions. The sizes of the obtained molded articles are measured. The condition under which an article having the highest size precision, among the measured values, was molded is selected. The injection molding apparatus and accompanying facilities are set in accordance with the selected molding condition. As a result, the setting operation of the optimum condition requires a large amount of labor and time.
Especially, since decisive information necessary for setting the large number of parameters (the temperature of the plasticizing cylinder, the temperature of the hot runner, the preset temperature of the cooling water temperature controller, the injection speed, the injection pressure, the dwell pressure of the mold, a time during which the dwell pressure should be maintained, the cooling time, the mold opening/closing speed, the control speed, operation, and position of the automatic molded article removing unit, and the like) of the molding condition to appropriate values cannot be obtained, the respective parameters are empirically set in accordance with a trial-and-error manner.
(3) In the conventional molding system, although the temperature of the coolant before supplied to the mold can be obtained from the preset temperature of the cooling water temperature controller, the temperature and flow rate of the coolant actually flowing in the branching lines in the mold cannot be obtained. Accordingly, when a molded article is actually removed from the mold, the temperature distribution of the entire molded article cannot be obtained, and the correspondence between the coolant used for temperature control in the mold and the temperature distribution of the molded article cannot be obtained. As a result, even when the molded article has a partial deformation due to a difference in partial heat shrinkage caused by the non-uniform temperature distribution of the molded article in the cooling process, no practical countermeasures can actually be taken to search the cause of the deformation or to prevent the deformation. As an effective countermeasure, at most merely the cooling time is prolonged to maintain the thermal uniformity of the molded article in the mold.
(4) The main causes of the deformation of the molded article in injection molding are the non-uniformity in the density distribution of the resin when filled in the mold and the non-uniformity of the temperature distribution of the molded article in the mold during the cooling process after filling. In other words, regarding the density distribution of the resin in the mold, the higher the density, the smaller the amount of shrinkage; the lower the density, the larger the shrinkage.
For this reason, it is assumed that a local stress is generated in the molded article due to the difference in local shrinkage, thereby causing deformation. Regarding the temperature distribution, a portion where the temperature is low reaches the setting temperature of the resin faster than other portions, and causes shrinkage in the mold in accordance with the thermal expansion coefficient of the resin. A portion where the temperature is high causes similar shrinkage after this to pull the portion which is set in advance, and causes local stress. When the molded article is to be removed from the mold, this portion causes deformation. In this case, however, the practical cause for the shrinkage cannot be obtained, and no effective countermeasures are taken.
(5) In the conventional molding system, the preset values of the injection molding apparatus and cooling water temperature controller can be selectively set as daily management and molding condition management performed after a change in procedure (e.g., mold exchange) of the molded article. In the conventional molding system, however, management concerning whether or not the actual molding states (the flowing state of the resin in the mold, the pressure distribution, the temperature distribution, the temperature and flow rate of the coolant, and the like) are reproduced in accordance with the required condition cannot be performed.
Accordingly, when, e.g., an error occurs in connection of the cooling water supply pipe which is to be connected to the mold, the line to flow the coolant therethrough clogs, and the capability of the heater for heating the plasticizing cylinder of the injection molding apparatus is changed and thus the actual mold temperature, molded article removing temperature, the viscosity of the resin, flowability of the resin, and the pressure distribution are changed, these changes cannot be instantaneously noticed, and countermeasures for them are delayed.
(6) In the conventional molding system, usually, an automatic molded article removing machine is operated based on a mold open completion (so-called mold opening) signal from the injection molding apparatus. A hand is inserted in the mold to hold the molded article, and thereafter the hand is retracted from the mold. The injection molding apparatus awaits to perform a mold closing operation until it receives a retreat completion signal from the automatic molded article removing machine. It is pointed out that an injection molding apparatus of this type has a considerably long loss time after it receives a mold closing signal until it starts an actual mold closing operation.
(7) In the conventional molding system, the automatic molded article removing machine is used to remove the molded article from the mold, as described above. An air cylinder type chuck operated by compressed air supplied in the factory is used as the hand of the removing machine. However, when molding is performed in one injection molding apparatus while sequentially exchanging a plurality of molds, every time a mold is exchanged, the chuck must be exchanged, and the chucking force and the like of the chuck must be adjusted. When an error occurs in this chuck adjustment (e.g., the chucking force), a molded article having an abnormal shape may be obtained. When the molded article removing temperature is high, the molded article is unexpectedly deformed by the chucking force acting on the molded article. As a result, the size precision of the molded articles varies, or a defective article is obtained.
With the conventional injection mold shown in FIG. 66, a sink is formed in the vicinity of the gate 39 of the molded article. With the conventional mold shown in FIG. 68, so-called stringing occurs when the mold is opened. In the mold shown in FIG. 66, a countermeasure may be taken, e.g., the thickness of the portion of the molded article around the gate 39 is decreased. Then, however, a flash or a silver stream occurs at a portion of the molded article corresponding to the vicinity of the gate 39, thus degrading the outer appearance. A flash is caused when the resin temperature is increased more than necessary to decompose the resin or to evaporate or decompose an additive in the resin. A silver stream is caused when the resin temperature is increased more than necessary to evaporate or decompose the water content in the resin, thereby hydrolyzing the resin or evaporating the additive in the resin. These defects pose a serious problem when, in particular, an outer surface of a plastic molded article that requires high precision and a good appearance is to be molded.
In FIGS. 70 and 73, when the columnar or cylindrical boss 150 is integrally formed by injection molded, several problems arise.
More specifically, in spite that the molded article 38A is cooled, after molding, to a temperature that enables releasing from the mold, the boss 150 and the base portion 202 are not sometimes cooled to the temperature required for releasing from the mold. Then, the molding cycle (cooling step) must be prolonged, leading to a decrease in productivity.
A recess may be formed in an fitting surface 200 of the boss 150 due to a difference in temperature drop among portions of the molded article. This degrades fitting precision or causes formation of a recess in a surface of the molded article main body 38A opposite to the boss forming portion, thereby degrading planar precision and outer appearance. As shown in FIGS. 74 and 75, when a recessed hole is formed in the boss, a decrease in strength of the boss 150 becomes a problem. Furthermore, on the side of the mold, when the position of the boss is to be changed, a female die portion of the boss 150 and a male portion forming the recessed hole 208 must be corrected simultaneously. Both the female and male dies must be altered, and alignment of the female and male dies becomes difficult, leading to an increase in cost.